The nature of a POP3 account is that it retrieves the emails from the Email Server and deletes their existence from there. So, yes, you could delete both but you would probably lose all your emails.

There are several possible ways around this but you should definitely back up all your profiles before trying anything.

It makes no sense to me that creating the IMAP account would have caused any mail duplication at the server. Did you also copy mail from the POP account to the IMAP account in Thunderbird maybe?

What is that weird "POP" folder you have under the Inbox of the POP account and the Trash of the IMAP? You must have created that folder yourself if it appears in the POP account…

Anyway, deleting the IMAP account should be no problem, but deleting the POP account, if you choose to delete both info and data, would cause you to lose all the mail downloaded by that account that's no longer on the server…

It makes no sense to me that creating the IMAP account would have caused any mail duplication at the server. Did you also copy mail from the POP account to the IMAP account in Thunderbird maybe? What is that weird "POP" folder you have under the Inbox of the POP account and the Trash of the IMAP? You must have created that folder yourself if it appears in the POP account… Anyway, deleting the IMAP account should be no problem, but deleting the POP account, if you choose to delete both info and data, would cause you to lose all the mail downloaded by that account that's no longer on the server…

I tried moving the e-mails around several which ways when I first opened the IMAP account, since it seemed to have only brought over a few of the e-mails. (I guess that's because some of the mail was no longer on the server?)

The POP folders you mention, I am quite certain, I did not create. I don't know why they appeared.

The nature of a POP3 account is that it retrieves the emails from the Email Server and deletes their existence from there. So, yes, you could delete both but you would probably lose all your emails. There are several possible ways around this but you should definitely back up all your profiles before trying anything.

I am thoroughly confused now.

I initially (years ago) when setting up the account in Thunderbird made it POP, for whatever reason - I assume I had some reason to think that was the right thing to do, like that's what the computer said it was, or whatever - but recently I have noticed that some e-mails weren't coming through in Thunderbird, but were coming through in Outlook webmail. Internet advice said that this was something to do with POP/IMAP, so I created an IMAP account in Thunderbird and sure enough the missing e-mails arrived.

So my assumption now is that on the Outlook servers the e-mail account was always IMAP and I'd mistakenly set it up as POP in Thunderbird, thus missing various e-mails - ergo, all the e-mails should still be on the Outlook.com servers, and I can delete the accounts in Thunderbird (and start a new one) that should download all my e-mails.

1. Where am I going wrong? 2. Would the simplest thing be to start afresh with Outlook desktop?

So my assumption now is that on the Outlook servers the e-mail account was always IMAP and I'd mistakenly set it up as POP in Thunderbird

No, that's a wrong assumption. It's common for email providers to allow both POP and IMAP access. They're not incompatible and you may even use both at the same time. You wouldn't have been able to "mistakenly" set it up as POP.

all the e-mails should still be on the Outlook.com servers

That's another wrong (and very dangerous) assumption. What's on the server is what you can see with IMAP or webmail (should be the same in both cases). What you cannot see with either is not there…

I can delete the accounts in Thunderbird (and start a new one) that should download all my e-mails.

What I said in my previous reply. For the IMAP account yes, for the POP account NO. Any messages stored locally in your POP account and not on the server are stored locally in that account and nowhere else. If you delete the POP account from Thunderbird and choose to delete both account info and message data without having made a backup first, you'll lose all the messages stored there that are not on the server…

Thanks, but I remain confused.

I won't try to understand everything, though, I just want to get the e-mails working. (Now it's telling me I can't send e-mails for some reason.) I'm thinking it will be easier to start from scratch. Suppose I backed up the POP account, exported the e-mails to some file or whatever - could I then import those e-mails into an IMAP account?

(And could I import them into another program? If I'm starting from scratch, why not switch back to Outlook?)

I won't try to understand everything

In this case, you should. The differences between POP and IMAP accounts are perfectly understandable and have a direct impact in how Thunderbird manages your mail both locally and at the server. You may easily get things wrong if you don't really understand how POP and IMAP works. The following article may help:

https://support.mozilla.org/kb/difference-between-imap-and-pop3

Suppose I backed up the POP account, exported the e-mails to some file or whatever - could I then import those e-mails into an IMAP account?

Yes, but it would be easier to move your mail out of the account folders into custom folders you may create under Local Folders. Mail placed there is stored locally on your computer independently of any mail account you may have.

And you may always move/copy mail from any folder locally stored on your computer to a folder in your IMAP account to make it available on the server.